The flexible circuit industry requires adhesives for polyimide film and metal foils (usually copper) which can withstand elevated temperatures and a variety of harsh solvents and chemicals. During the many preparation and processing steps for circuit manufacture, these solvents and chemicals can cause an adhesive to swell leading to blister formation and/or delamination, which results in reduced circuit yields. The application of heat, such as in soldering, can similarly cause circuit failures.
U.S. Pat. Nos. 3,822,175, 3,900,662 and 3,728,150 disclose crosslinkable acrylic adhesive compositions for bonding polyimide films. Copper-polyimide laminates made using such adhesive compositions have good peel strength, are stable to soldering temperatures and are resistant to chemicals typically used in circuit manufacture.
It is sometimes desirable, however, to use adhesive compositions which have additional properties, such as flame retardancy, electrical conductivity, thermal conductivity, etc. Such special adhesive compositions are usually obtained by the addition of functional fillers to the basic composition. However, fillers tend to make the adhesive more brittle which adversely affects the peel strength of laminates made with the filled adhesive. This brittleness can be overcome somewhat by the addition of plasticizers, however these frequently are attacked by processing chemicals and generally are not stable at soldering temperatures, which can result in blister formation.
Thus, there is a need for an adhesive for polyimide film which has special functional properties without sacrificing peel strength, chemical resistance or thermal stability.